In most types of footwear, especially athletic shoes, the lower or underfoot portion of the shoe comprises a midsole which is directly attached to the shoe upper portion. A wear surface is fixed to the bottom of the midsole. The purpose of the wear surface is to resist wear as the ground is contacted.
The midsole is designed primarily to provide stability for the foot while attenuating shock. In running and walking, the foot makes initial contact with the ground surface on the lateral aspect of the rearfoot area. At initial contact, the foot and ankle sustain the greatest shock as the weight of the body is shifted to the heel of one foot. Immediately after initial contact, the lower limb experiences internal rotation and the foot pronates at the subtalar joint to initiate the midstance phase of the gait until the entire foot rests on the ground. At this time the lower limb experiences external rotation and the motion of the foot is supination. The final phase of the gait cycle is the propulsive phase during which the foot is supinating to provide a rigid lever for exerting the force of the leg on the ground surface to propel the body forward.
The footwear, in serving as the contact member between the foot and ground, ideally must first be a shock attenuator and stabilizer for the heel during the contact phase of the gait. Thereafter during the midstance phase the shock attenuation function continues but to a lesser degree while foot stabilizing increases in importance. During the propulsive phase the stability of the foot remains important with the shoe also now serving to transmit the propelling force from the foot to the ground surface. It can be seen that the shoe midsole ideally must be pliable during the contact and midstance phases but should be at least semi-rigid during the propulsion phase while always providing both lateral and medial stability to protect the ankle.
Present shoe midsoles have represneted a compromise design in an attempt to accommodate the various phases of running and walking. To reduce the shock of initial contact with the ground it is advantageous to provide a more pliable shoe midsole. However, the more pliable midsole provides less stability to the foot, that is, it allows the foot to roll over to one side or the other more easily, an action that can easily result in injury to the foot or ankle. Such injuries are even more frequent when running on uneven ground and solid or hard surfaces.
It is the purpose of the present invention to provide greater shock attenuation in a shoe while maintaining stability and foot control throughout the weight bearing phases of gait.